1) Lower the CPU/board frequency. As any electronic device PSP emmits some EM frequencies, that seems to interfere with PSP-290 operation. So lowering the CPU frequency results in generating less EM radiation and therefore slightly better performance of the GPS unit.
2) Use a DYI cable to distance the unit from PSP.
Following mini-B usb extension cable can be tweaked to do this job: http://pc-mobile.net/cmue.jpg
One still needs to pull an extra wire to connect the power port to the left of the USB socket with left power pin on the GPS receiver.
Following thread has more details on the hookup with some photos: http://www.dcemu.co.uk/vbulletin/showthread.php?t=62604
3) Some people reported better GPS performance after shielding the back and top of their PSPs with metal cover or aluminum foil.
4) Charging battery. Having a full charge seems to make a small difference, according to some reports.
5) Changing PSP or/and PSP-290. It seems that different PSP batches have different EM noise levels. Japanese PSPs seem to do slightly better than others. Also, from my tests, out of 2 PSP-290 units that I own, 1 always seem to do better in terms of sensitivity and presision. So exchanging the unit may help a bit too...
6) Passive cellphone booster stickers - although this does not sit well with the stuff I learned from college physics, some folks tend to believe that sticking one of those stickers on the unit's buttom seem to improve the reception. Feel free to try this theory, if you can get one of those stickers cheap or for free. If not, sticking some aluminum foil to the back of the unit would probably make the same effect (if any at all)
7) Upgrading firmware - This in theory may help a bit, but... SCE has fixed most of the GPS module bugs somethere around FW 3.03, so upgrading to higher firmware probably would not noticably improve the performance.